The Doppler Effect is well known in the art. The Doppler Effect arises when a source or an object, emitting waves that propagate in a medium, is in motion relative to an observer or receiver. The Doppler Effect likewise arises as a source or an object, emitting waves that propagate in a medium, is still and an observer is moving relative to the source/object. The Doppler Effect is the change in frequency of a wave for an observer moving relative to the source of the wave. One commonly known example is when a vehicle sounding a siren approaches, passes and travels away relative an observer. As the vehicle approaches, the received frequency at the observer is higher compared to the emitted frequency from the vehicle. Just as the vehicle passes the observer, the frequency at the observer is identical to the emitted frequency. As the vehicle travels away from the observer, the frequency is lower at the observer compared to the emitted frequency from the vehicle.
This phenomenon affects all waves that propagate in a medium. Of course both the observer and the source emitting the waves may be in movement relative each other. In case both the observer and the source are moving, the total Doppler Effect may therefore result from both the movement of the source and the observer. Also the movement of the medium may affect the Doppler Effect.
The Doppler Effect, also called Doppler shift or Doppler frequency, is a factor that needs to be considered in mobile communication systems or applications in wireless mobile communications. It is generally important to have or obtain an accurate estimate of the maximum Doppler frequency in a mobile communication system, e.g. in order to receive and to process received radio signals properly. The maximum Doppler frequency provides information about the rate of change of the channel. Having an accurate estimation of the rate of change of the channel may aid the system in compensating for the change or adjusting different parameters of the channel in question.
The very essence of a mobile communication system is that the user of a mobile telephone or other wireless communication apparatus or device may move about freely, within radio coverage areas. As a user moves around, the communication channel may change. The communication channel is affected by the distance between the communication apparatus and the serving access node or base station, by objects, such as e.g. buildings, that are present in the area which is served by the access node or base station and so on. Also, a user may walk around at relatively low speed, or be situated in a car, bus, train or the like travelling at relatively high speed. This will also affect the Doppler Effect and also the channel will change more rapidly.
There are many Doppler frequency estimation methods. Some common methods are autocorrelation based techniques. These are of low complexity and have therefore been extensively used. However, many autocorrelation functions are nonlinear functions and it is difficult to accurately extract the maximum Doppler frequency from them.